Quantum dots are small particles of matter that typically have cross-sectional dimensions of less than about ten nanometers. Quantum dots generally are small enough that the addition of an electron to the quantum dot or the removal of an electron from the quantum dot changes the properties of the quantum dot in some detectable way. Quantum dots have been employed in several areas of technology. For example, quantum dots have been used in lasers, light emitting diodes and infrared detectors. Extensive research is currently being conducted to identify additional applications for quantum dots in these areas of technology, and to determine the utility of quantum dots in other areas of technology including electronics, optoelectronics, telecommunications, and biotechnology.
The performance or behavior of a device that includes quantum dots generally is at least partially dependent upon the size and shape of the individual quantum dots, and upon the spacing between the quantum dots. Therefore, the ability to produce a useful and functional device that employs quantum dots is at least partially a function of the ability to produce quantum dots having well controlled size, shape, and spacing. It may be desirable or necessary to control the size, shape and spacing to within a few nanometers. These extremely small dimensions and extremely tight tolerance requirements make the production of quantum dots very difficult.
Various techniques and methods for producing quantum dots have been presented in the art. Such techniques include chemical synthesis, molecular beam epitaxy, chemical vapor deposition, and gas condensation techniques, such as thermal evaporation, sputtering, electron beam evaporation, or laser ablation. Self-assembly techniques have also been employed to produce quantum dots.
These techniques for producing quantum dots that have been presented in the art generally suffer from at least one of two problems. First, many techniques cannot control the size, shape, and spacing of the quantum dots in a sufficiently precise manner. Second, many techniques are slow or expensive and, therefore, are not economically suitable for mass producing quantum dots or devices that include quantum dots. As a result, there is a need in the art for methods that allow for the production of quantum dots having well controlled size, shape, and spacing. Furthermore, there is a need in the art for methods that facilitate the mass production of such quantum dots in an economically efficient manner.
One area of technology in which quantum dots may be employed is nano-enhanced Raman spectroscopy (NERS).
Raman spectroscopy is a well-known technique for analyzing molecules or materials. In conventional Raman spectroscopy, high intensity monochromatic radiation provided by a radiation source, such as a laser, is directed onto an analyte (or sample) that is to be analyzed. This radiation may be referred to as the incident radiation. In Raman spectroscopy, the wavelength of the incident radiation typically is varied over a range of wavelengths within or near the visible region of the electromagnetic spectrum. A majority of the photons of the incident radiation are elastically scattered by the analyte. In other words, the scattered photons have the same energy, and thus the same wavelength, as the incident photons. However, a very small fraction of the photons are inelastically scattered by the analyte. Typically, only about 1 in 107 of the incident photons are inelastically scattered by the analyte in conventional Raman spectroscopy. These inelastically scattered photons have a different wavelength than the incident photons. This inelastic scattering of photons is termed “Raman scattering”. The Raman scattered photons can have wavelengths less than, or, more typically, greater than the wavelength of the incident photons.
When an incident photon collides with the analyte, energy can be transferred from the photon to the molecules or atoms of the analyte, or from the molecules or atoms of the analyte to the photon. When energy is transferred from the incident photon to the analyte, the Raman scattered photon will have a lower energy and a corresponding longer wavelength than the incident photon. These Raman scattered photons having lower energy than the incident photons are collectively referred to in Raman spectroscopy as the “Stokes radiation.” A small fraction of the analyte molecules or atoms can be in an energetically excited state when photons are incident thereon. When energy is transferred from the analyte to the incident photon, the Raman scattered photon will have a higher energy and a corresponding shorter wavelength than the incident photon. These Raman scattered photons having higher energy than the incident photons are commonly referred to in Raman spectroscopy as the “anti-Stokes radiation.” The Stokes radiation and the anti-Stokes radiation collectively are referred to as the Raman scattered radiation or the Raman signal.
The Raman scattered radiation is detected by a detector that typically includes a wavelength-dispersive spectrometer and a photomultiplier for converting the energy of the impinging photons into an electrical signal. The characteristics of the electrical signal are at least partially a function of both the energy of the Raman scattered photons as evidenced by their wavelength, frequency, or wave number, and the number of the Raman scattered photons as evidenced by the intensity of the Raman scattered radiation. The electrical signal generated by the detector can be used to produce a spectral graph illustrating the intensity of the Raman scattered radiation as a function of the wavelength of the Raman scattered radiation. Analyte molecules and materials generate unique Raman spectral graphs. The unique Raman spectral graph obtained by performing Raman spectroscopy can be used for many purposes including identification of an unknown analyte, or determination of physical and chemical characteristics of a known analyte.
Raman scattering of photons is a weak process. As a result, powerful, costly laser sources typically are used to generate high intensity incident radiation to increase the intensity of the weak Raman scattered radiation for detection. Surface-enhanced Raman spectroscopy (SERS) is a technique that has been developed to enhance the intensity of the Raman scattered radiation relative to conventional Raman spectroscopy. In SERS, the analyte molecules typically are adsorbed onto or placed adjacent to what has been referred to as a SERS-active structure. SERS-active structures typically include a metal surface or structure. Interactions between the analyte and the metal surface cause an increase in the intensity of the Raman scattered radiation. The mechanism by which the intensity of the Raman scattered radiation is enhanced is not completely understood. Two main theories of enhancement mechanisms have been presented in the literature: electromagnetic enhancement and chemical enhancement.
Several types of metallic structures have been employed in SERS techniques to enhance the intensity of Raman scattered radiation that is scattered by analyte molecules adjacent thereto. Some examples of such structures include electrodes in electrolytic cells, metal colloid solutions, and metal substrates such as a roughened metal surface. For example, it has been shown that adsorbing analyte molecules onto or near a specially roughened metal surface made from gold or silver can enhance the Raman scattering intensity by factors of between 103 and 106.
Recently, Raman spectroscopy has been performed employing randomly oriented nanostructures, such as nanometer scale needles, particles, and wires, as opposed to a simple roughened metallic surface. This process will be referred to hereinafter as nano-enhanced Raman spectroscopy (NERS). The intensity of the Raman scattered photons from a molecule adsorbed proximate to nanoparticles or structures that include nanoparticles can be increased by factors as high as 1016. At this level of sensitivity, NERS has been used detect single molecules. Detecting single molecules with high sensitivity and molecular specificity is of great interest in the fields of chemistry, biology, medicine, pharmacology, and environmental science. However, it is unknown what configurations, including size, shape and spacing, of metallic nanoparticles will enhance the intensity of Raman scattered radiation most effectively. In addition, it has proven very difficult to fabricate metallic nanoparticles having well controlled size, shape and spacing.
Accordingly, there is a need for a method that can be used to quickly and cost-effectively produce quantum dots and nanoparticles having well controlled size, shape, and spacing that can be used to enhance the intensity of Raman scattered radiation scattered by an analyte in the vicinity of the quantum dots while performing NERS.